Human Rights Watch on Friday accused the governments of the United States and El Salvador of “a grave violation of international human rights law” over the deportation more than 230 Venezuelan nationals by the Trump administration to a megaprison in El Salvador last month.

The actions taken against the deportees constitute both enforced disappearances and arbitrary detention, according to a statement from the group released Friday.

“The cruelty of the U.S. and Salvadoran governments has put these people outside the protection of the law and caused immense pain to their families,” said Juanita Goebertus, Americas director at Human Rights Watch.

The group said that since their removal, the Venezuelans “have been held incommunicado” and that the United States and Salvadoran officials have not released a list of the people who were removed, though CBS News last month published a list of names the outlet obtained.

The administration invoked the Alien Enemies Act, a rarely used law that gives the president broad authority to detain or deport non-citizens during times of war, to justify dozens of the deportations—triggering a fierce legal battle.

The Trump administration invoked the Alien Enemies Act in response to an alleged “invasion” by “Tren de Aragua,” a Venezuelan gang, but the government has produced scant evidence that the people removed had ties to Tren de Aragua. One hundred and one of the deportees were removed under regular immigration procedures.

According to Human Rights Watch, the Salvadoran government has failed to offer a legal basis for detaining the Venezuelan deportees and has not indicated when or whether they will be released.

“It appears that their detention is wholly arbitrary and potentially indefinite; a grave violation of El Salvador’s human rights obligations,” the group said.

Enforced disappearance, according to Human Rights Watch, is when officials deprive someone of their liberty and then conceal the fate or whereabouts of the disappeared person. The violation is “especially serious” because it means they are outside the protection of the law.

The group is calling on U.S. authorities to publicly identify the Venezuelans who were removed to El Salvador and is urging the Salvadoran government to “confirm their current whereabouts, disclose whether there is any legal basis for their detention, and allow them contact with the outside world.”

The statement from Human Rights Watch also detailed the struggle that family members of the deported individuals have faced getting information about them.

The group has interviewed 40 relatives of people “apparently” removed to El Salvador, and all of them told Human Rights Watch that U.S. immigration authorities initially informed their relatives, who were in U.S. immigration detention, that they would be sent to Venezuela. They were not told they would be sent to El Salvador.

“Nobody should be forced to piece together bits of information from the media or to read into the authorities’ silence to find out where their relatives are being held,” Goebertus said. “Salvadoran authorities should urgently disclose the names and locations of all detainees transferred from the US, and allow them to contact their families.”

In addition to the Venezuelans who were deported in March, the Trump administration also deported a smaller number of Salvadoran nationals. U.S. Immigration and Customs Enforcement has admitted that one of the men sent to El Salvador was deported in “error.” On Thursday, the Supreme Court instructed the Trump administration to take steps to retrieve the man it had wrongly deported.